Underground passages are prevalent in developed areas and provide passageways for everything from fiber optic data lines, to sewage, to electrical power transmission lines. Workers require access to the underground passageways to perform routine maintenance and to correct defects that may occur to the utility passing within. Thus, the manhole cover was created to close off the passageways and still provide a means of ingress and egress to the passageways.
A manhole cover must be durable, strong, and relatively inexpensive. Durability is required as many cover applications are exposed to the environment and associated climatic conditions. Strength is desired because many applications place the manhole cover into high traffic situations where the cover is required to repeatedly support a large amount of weight, for example, a passing garbage truck on a city street. Cost is a driving force behind most all cover designs and inexpensive manhole covers are preferred, especially given the volume of manhole cover applications. As a result, the type of materials used as well as the type and amount of secondary manufacturing operations employed in the design and production of manhole covers is limited.
Manhole covers come in various shapes and sizes. Manhole covers may be circular or any other shape that prevents a worker from dropping the cover into the manhole it was designed to cover. Covers range in size depending upon the application and can be very heavy. Most manhole covers are big enough to permit human passage, perhaps a meter or more across. Furthermore, due to the durability, strength, cost, and application requirements discussed above, manhole covers are typically cast iron and weigh between thirty to over one-hundred and thirty kilograms.
The size and weight of many manhole covers combine to create a heavy, awkward cover and give rise to several potential hazards and problems. Workers having to remove and lift larger covers are susceptible to accidental injuries should the cover slip and land upon a hand, leg, foot, or other body part. Repeated lifting of heavy covers may lead to chronic physical ailments requiring painful and expensive rehabilitation. The demanding physical requirements imposed by the size and weight of some covers limits the pool of workers capable of individually manipulating a cover. In turn, this requires that more than one worker be deployed to accomplish a task that, but for the manipulation of a manhole cover, could be accomplished by a single worker.
Attempts have been made to reduce the stresses placed on a worker in manipulating a manhole cover while meeting the durability, strength, and cost requirements of each application; however, room still exists for improvement.